Robot gripper mechanism and method for sampling nuclear fuel pellets

ABSTRACT

A gripper mechanism for sampling nuclear fuel pellets at a predetermined depth in a stack of pellets includes a housing and gripping fingers movably mounted to the housing. The fingers are preferably three relatively thin, arcuate-shaped, blade-like fingers angularly displaced from one another for grasping the pellet at spaced locations about its circumference. The fingers have lower facing surfaces which are brought adjacent to one another for engaging and grasping a pellet when the fingers are moved toward their closed position and retracted remote from one another for disengaging and releasing the pellet when the fingers are moved toward their opened position. Guide members interconnecting the housing with a movable support structure, together with coil springs received about the guide members and extending between the housing and support structure, resiliently and yieldably mount the housing and fingers for movement toward the stack of pellets along a generally linear path ahead of the support structure as the support structure is moved toward the stack. Concurrently the yieldable mounting permits the housing to rock slightly about an axis extending transverse to the linear path for facilitating a burrowing-type movement of the gripping fingers into the stack of pellets to be sampled. Also, a proximity switch detects when the fingers are closed and a load transducer senses when an overload condition has been reached.

CROSS REFERENCE TO RELATED APPLICATION

Reference is hereby made to the following copending U.S. patentapplication dealing with related subject matter and assigned to theassignee of the present invention: "Manufacturing Automation System ForNuclear Fuel Rod Production" by Francis Cellier et al, assigned U.S.Ser. No. 702,520 and filed Feb. 19, 1985, now U.S. Pat. No. 4,687,605.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to automated article handlingand, more particularly, is concerned with a gripper mechanism on a robotmanipulator arm and the method of operation thereof for removingindividual nuclear fuel pellet samples from different depths in a stackof pellets in a sintering boat.

2. Description of the Prior Art

Manufacture of nuclear fuel in pellet form conventionally involvesblending radioactive powder material to the desired chemical compositionand then fabricating it into pellets. The properly blended powder ismade into pellets by first forming it into slugs, then granulating theslugs and mixing a lubricant with the granulates, and lastly pressingthe lubricated granulates into green pellets. The green pellets arecompactly stacked in layers in receptacles called furnace boats and fedinto a sintering furnace where high temperatures sinter the pellets in ahydrogen atmosphere to achieve the required density and microstructure.

However, it is necessary to select representative pellets from thefurnace boats immediately after the sintering operation. It is currentlyrequired that sample pellets representing top, middle and bottom layersbe taken at random. The sampled pellets are examined to determinedensity and other characteristics prior to releasing the product forfurther processing.

Current practice is to select the samples manually; however, sinteredpellets are hard, relatively heavy, ceramic cylinders with sharp edges.When confined to a container, such as a furnace boat, the pellets tendto form a compact array that is difficult to penetrate. Obtainingsamples manually from anywhere but the surface layer is physicallydifficult, requiring the operator to thrust or "worm" his or her fingersdown through the massed pellets.

Also, steps are underway to automate pellet manufacturing lines, such asgenerally described in the application cross-referenced above, whichwill make manual access to internal pellet samples even more difficult.For example, automated boat loaders are being implemented to achieve agreater compaction of pellets within the boat by arranging the pelletsin interlocking vertical stacks. Further, larger and deeper boats arebeing used. Conventional boat designs accommodate approximately 1000pellets, whereas the new boats used in automated operations holdapproximately 6000 pellets. Accordingly, a greater number of sampleswill be required from each boat, but the difficulty in obtaining manualaccess to the lower layers will be compounded.

Mechanisms for automated handling of articles of various sizes andshapes are known in the prior art. Representative of the prior art arethe mechanisms disclosed in U.S. Pat. Nos. to Panissidi (3,583,751),Mack (4,211,123), O'Neil (4,234,223), Birk et al (4,266,905), Thomson etal (4,273,506), Wood (4,348,044) and Higgins (4,421,451). While many ofthese mechanisms achieve their objectives under the range of operatingconditions for which they were designed, most would seem to beunsuitable in terms of the versatility of their operating capabilitiesfor sampling nuclear fuel pellets from a compact layered stack thereofcontained in a boat.

Consequently, a need still exists for a suitable automated mechanism andtechnique to sample nuclear fuel pellets from layers at different depthswithin the furnace boat in a consistent, repeatable way without damagingthe pellets contained therein.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a robot gripper mechanism and method ofsampling nuclear fuel pellets designed to satisfy the aforementionedneeds. The gripper mechanism of the present invention is part of anautomated sub-system incorporated in the manufacturing system disclosedin the cross-referenced application for automatic fuel rod production.Specifically, the gripper mechanism and gripper sampling method inconjunction with a robotic manipulator arm, such as a PUMA 560 robot,automatically select individual nuclear pellets from various locationsand depths within the stacked, layered load of pellets in the sinteringfurnace boat. Each pellet is then moved through successive stations ofan interfacing automatic density measurement sub-system by the samerobot.

Advantageously, the gripper mechanism and sampling method of the presentinvention eliminates the human factor from the sampling process whichassures the integrity and consistency of a difficult sampling operation.Also eliminated are the hazards associated with manual sampling wherethe operator is required to work his or her hand down through severalinches of sharp, radioactive ceramic pellets to secure samples frommiddle and bottom layers. Consonant with functional requirements of theoverall automated fuel rod manufacturing system, the present inventionprovides unattended automated sampling which improves productivity,permits safe selection of greater sample quantities from larger boatsbeing machine loaded to greater load densities without emptying thecontents for manuals sampling, and maintains sample-to-boat tacking fordata analysis and product traceability.

Accordingly, the present invention sets forth a gripper mechanism forlocating and grasping a nuclear fuel pellet at a predetermined depth ina stack thereof. The gripper mechanism comprises: (a) a housing; (b)gripping means mounted to the housing and being movable toward and awayfrom respective closed and opened positions for respectively engagingand disengaging the pellet to correspondingly grasp and release thesame; (c) actuating means mounted to the housing and coupled to thegripping means, the actuating means being operable for moving thegripping means between its closed and opened positions; (d) means fordetecting when the gripping means is disposed at its closed position;and (e) means supporting the housing and the gripping means mountedthereto for movement toward and away from the stack of pellets to besampled for moving the gripping means into and from the stack, thesupporting means also resiliently and yieldably mounting the housing soas to allow concurrent lateral rocking of the housing as the grippingmeans in moved into and from the stack of pellets.

More particularly, the gripping means includes a plurality of grippingfingers movably mounted to and depending from the housing. The fingersare movable between the closed and opened positions and have respectivelower engaging surface portions and have respective lower engagingsurface portions. The engaging surface portions are brought adjacent toone another for engaging a pellet therebetween when the fingers aremoved toward their closed position and are retracted remove from oneanother for disengaging a pellet when the fingers are moved toward theiropened position.

Further, means is provided for sensing the load applied to the stack ofpellets by the gripping means. The sensing means in the form of aload-sensing transducer is capable of generating a signal indicatingthat an overload condition has been reached. Additionally, the meansresiliently mounting the housing includes a support structure beingmovable toward and away from the stack of pellets, a plurality of guidemembers connected to one of the housing and the support structure andslidable relative to the other thereof, and a plurality of coil springs.Each coil spring is received about one of the guide members and extendsbetween the housing and support structure. The springs are resilientlyyieldable for moving the housing and gripping means toward the stack ofpellets along a generally linear path ahead of the support structure asthe support structure is moved toward the stack, while concurrentlypermitting the housing to rock about an axis extending transverse to thelinear path for facilitating burrowing-type movement of the grippingmeans into the stack of pellets.

The actuating means is a power transmitting cylinder mounted to thehousing and having an extendible and retractible piston rod and aspindle mounted to its terminal end. The spindle is coupled to thegripping means for moving the gripping means toward its closed andopened positions upon retraction and extension of the piston rod intoand from the cylinder. The detecting means is a proximity switch mountedon the housing for detecting the position of the piston rod.

The present invention also relates to a method of sampling a stack ofnuclear fuel pellets at a predetermined depth in the stack, comprisingthe steps of: (a) locating a plurality of angularly displaced grippingfingers above the stack; (b) opening lower end portions of the grippingfingers; (c) burrowing the gripping fingers into the stack until thelower end portions thereof are located at the predetermined depth in thestack; (d) closing the lower end portions of the fingers to grasp apellet located at the predetermined depth; and (e) withdrawing thefingers upwardly from the stack of pellets. More particularly, theburrowing step includes: (i) lowering the fingers into the stack ofpellets toward the predetermined depth therein; (ii) rotating thefingers in a first direction relative to the stack and at a firstpredetermined speed concurrently as the fingers are being lowered intothe stack; (iii) pausing the lowering of the fingers; (iv) rotating thefingers in a second direction opposite to the first direction relativeto the stack and at a second predetermined speed faster than the firstspeed concurrently as the fingers are paused; and (v) repeating steps(i) through (iv) until the desired predetermined depth is reached. Also,during at least one interval when the fingers are paused, the fingersare rotated multiple times in opposite directions to stir the stack ofpellets so as to facilitate further lowering of the fingers into thestack.

These and other advantages and attainments of the present invention willbecome apparent to those skilled in the art upon a reading of thefollowing detailed description when taken in conjunction with thedrawings wherein there is shown and described an illustrative embodimentof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the following detailed description, reference will bemade to the attached drawings in which:

FIG. 1 is a perspective view of a robotic manipulator arm with the robotgripper mechanism of the present invention thereon grasping a samplenuclear fuel pellet taken from a compact, layered stack of pelletscontained in a sintering boat.

FIG. 2 is an enlarged elevational view of the gripper mechanism of FIG.1, showing the gripping fingers of the mechanism being disposed in aclosed position without a pellet being grasped therebetween.

FIG. 3 is a a bottom plan view of the gripper mechanism as seen alongline 3--3 of FIG. 2.

FIG. 4 is a view of the gripper mechanism similar to that of FIG. 2, butshowing the gripper mechanism in partially sectioned form.

FIG. 5 is a view of the gripper mechanism similar to that of FIG. 4, butshowing the gripping fingers of the mechanism being disposed in anopened position.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views of the drawings.Also, in the following description, it is to be understood that suchterms as "forward", "rearward", "left", "right", "upwardly","downwardly", and the like, are words of convenience and are not to beconstrued as limiting terms.

In General

Referring now to the drawings, and particularly to FIG. 1, there isillustrated a sampling station, generally designated 10, of an automatedfuel rod manufacturing system, such as generally disclosed in theapplication cross-referenced above. The sampling station 10 is locateddown-stream of the discharge end of a sintering furnace from which aboat 12 loaded with sintered pellets P, being contained in a tightlycompact, layered stack therein, is automatically transported by aconveyor 14 to the station 10. At the sampling station 10,representative ones of the sintered pellets P in the boat 12 arerandomly sampled from different depths of the boat in accordance with aprescribed sampling plan and their density and other characteristicsinspected.

The sampling is carried out by the robot gripper mechanism 16 of thepresent invention being mounted on the terminal end of a roboticmanipulator arm, generally indicated 18, located in the sampling station10 adjacent the conveyor 14. The gripper mechanism 16 is operable in asequence of gripper movements which constitute the steps of the pelletsampling method of the present invention. In one representative example,to operate the gripper mechanism 16 the manipulator arm 18, a multi-axesdevice such as a PUMA 560 robot arm, is combined with a suitablecontroller, such as a LSI-11 computer, external storage (floppy disk),programming device (teach pendant), CRT console and a four way pneumaticsolenoid gripper control. Since other arrangements of suitablecommercially-available components can be used to operate the grippermechanism 16, the aforementioned components associated with themanipulator arm 18, not comprising any part of the present invention,need not be illustrated nor described herein in order for one skilled inthe art to gain a complete and thorough understanding of the grippermechanism 16 and its mode of operation.

Robot Gripper Mechanism

Turning now to FIGS. 2-5, there is shown the preferred embodiment of therobot gripper mechanism 16 of the present invention. The grippermechanism 16, adapted to gently burrow into a compact stack of pellets Pand grasp one located at a predetermined depth in the boat 12, basicallyincludes a housing 20 and a plurality of elongated gripping fingers 22pivotally mounted at their upper ends 24 by pins 26 to spaced pairs oftabs 28 on the lower periphery of the housing 20.

More particularly, the gripping fingers 22 are preferably threerelatively thin, arcuate-shaped, blade-like fingers. As in the case ofthe pairs of tabs 28, the fingers 22 are angularly displaced generallyequally from one another, for example 120 degrees apart, for graspingthe cylindrical pellet P in the stack at three equidistantly spacedlocations about the circumference of the pellet. By their pivotalmounting to the tabs 28 about axes which extend generally parallel tovertical planes extending tangential to the circumference of thecylindrical pellet to be grasped, the gripping fingers 22 are thuspivotally movable between inner closed and outer opened positions,respectively depicted in FIGS. 4 and 5. The gripping fingers 22 haverespective lower ends 30 with generally vertical, pellet engagingsurfaces 32 facing toward one another. The finger surfaces 32 arebrought adjacent to one another for engaging and grasping a pellet P(see Rig. 1) therebetween when the fingers 22 are moved toward theirclosed position (FIGS. 3 and 4) and are retracted remote from oneanother for disengaging and releasing the pellet when the fingers 22 aremoved toward their opened position (FIG. 5).

For moving the gripping fingers 22 toward and away from their closed andopened positions, the gripping mechanism 16 includes actuating means inthe form of an air cylinder 34, although other suitable devices could beused, mounted to the housing 20 and coupled to the upper ends 24 of thegripping fingers 22. The air cylinder 34 has a power-transmittingcylinder portion 36 attached to the housing 20 and a piston rod 38mounted in the cylinder portion and extendible and retractable relativethereto. THe piston rod 38 has a circular disc or spindle 40 mounted toits lower terminal end 42 and coupled in inwardly facing arcuate notches44 formed in the upper ends 24 of the gripping fingers 22 inwardly fromthe location of pivot pins 26. The housing 20 is provided with air inletand outlet ports 46,48 in communication with the cylinder portion 36through which pressurized air is routed from any suitable source (notshown) for extending and retracting the piston rod 38. The grippingfingers 22 are concurrently pivoted toward and away from their closedand opened positions upon movement of the piston rod 38 relative to thecylinder portion 36 between retracted and extended positions,respectively seen in FIGS. 4 and 5.

For moving the housing 20 and gripping fingers 22 mounted thereon towardand away from the stack of pellets P contained in the boat 12, thegripper mechanism 16 includes a support structure in the form of amounting flange assembly 50, a plurality of connecting guide members 52and a plurality of resilient flexible members in the form of coilsprings 54. The mounting flange assembly 50 is connected to the outerend 56 of the manipulator arm 18 for rotation about a central axis A ofthe gripper mechanism 16, allowing the fingers 22 of the mechanism 16not only to be moved toward and away from the stack of pellets but alsorotated relative to the stack concurrently with such movement.

The guide members 52 are spaced apart preferably 90 degrees and extendbetween the flange assembly 50 and a mounting base 58 of the housing 20.The threaded bottom ends 60 of the guide members 52 are threaded intoholes 62 tapped in the housing mounting base 58. The guide members 52slidably fit through openings 64 in the flange assembly 50 which arespaced apart in alignment with the holes 62. The upper ends of the guidemembers 52 have enlarged diameter heads 66 which are sized to seat onledges 68 formed on the flange assembly 50 within the openings 64therein so as to suspend the housing 20 below the flange assembly.Further, each of the coil springs 54 is compressed and disposed aboutone of the guide members 52 and extends between the housing base 58 andthe flange assembly 50 so as to disposed the housing 20 at a maximumdistance from the assembly 50 with the guide member heads 66 seated onthe ledges 68. However, as would be expected, the coil springs 54 areindependently resiliently yieldable for allowing angularly displacedportions of the housing 20 to move toward the flange assembly 50 and insuch manner impart a lateral rocking motion to the housing about axeslocated within a plane extending transverse to the longitudinal axis Aof the gripper mechanism 16. Such mounting relationship of the housing20 to the flange assembly 50 allows the housing and gripping fingers 22to be moved by the arm 18 toward the stack of pellets, for instance,along a generally linear path ahead of the flange assembly 50 as thelatter is moved toward the stack while concurrently permitting thehousing 20 to rock slightly about axes within the plane extendingtransverse to the linear path, facilitating a burrowing-type movement ofthe gripping fingers 22 into the stack of pellets to be sampled.

The gripper mechanism 16 further includes means in the form of aproximity sensor or switch 70 mounted on the housing 20 for detectingwhen the gripping fingers 22 are disposed at their inner closedposition. Specifically, when the proximity switch 70 detects thepresence of a washer 72 which is mounted on the upper end 74 of thepiston rod 38, this indicates that the gripping fingers 22 are at theirclosed position.

Since it is extremely important not to damage any of the pellets duringlowering of the gripping fingers 22 into the stack, the grippermechanism 16 also incorporates means in the form of a load-sensingtransducer 76 mounted on the flange assembly 50 for sensing the loadapplied to the pellets by the fingers. The transducer 76 is capable ofgenerating a signal indicating that an overload condition has beenreached. In particular, the transducer 76 has circular disk portion 78of a generally planar shape and a central stem 80 received in a centralbore 82 of the mounting flange assembly 50 and secured therein by a setscrew 84 threaded into an internally threaded passageway 86 whichintersects with the central bore 82. The disk portion 78 of thetransducer 76 extends in generally transverse relation to the directionof movement of the gripper mechanism 16 toward and away from the pelletstack. The housing 20 has a generally planar leveling pad 88 mountedthereto above its mounting base 58 and also extending in generallytransverse relation to the direction of movement of the grippermechanism 16. Upon yielding of the coil springs 54, the leveling pad 88of the housing 20 will engage the planar disk portion 73 of thetransducer 76 and impose on the transducer the load being transmitted tothe gripping fingers 22 to move it into the pellet stack. The output ofthe transducer 76 will indicate whenever an unsatisfactory preselectedlevel of load is reached.

Nuclear Fuel Pellet Sampling Method

The above-described gripper mechanism 16 is used to sample a stack ofnuclear fuel pellets P generally at several different predetermineddepths in the stack while the same remains in the boat 12. Themanipulator arm 18 initially locates the angularly displaced grippingfingers 22 more or less directly above the stack, with their lower ends30 in opened position, as seen in FIG. 5. The manipulator arm 18 is thenoperated to burrow the gripping fingers 22 into the stack until theirlower ends 30 are located at the desired predetermined depth in thestack. Next, the air cylinder 34 in actuated to retract the piston rod38 and move the lower ends 30 of the fingers toward their closedposition in order to grasp a pellet located at the predetermined depthbetween the surfaces 32 on the fingers, such as seen in FIG. 1. If theproximity switch 70 does not detect the presence of the washer 72, thenone known that a pellet has been successfully grasped by the fingers.Finally, the manipulator arm 18 is operated to withdraw the fingers 22and sampled pellet upwardly from the stack of pellets.

More particularly, gentle burrowing of the fingers 22 is achieved asfollows. First, the fingers 22 are lowered into the stack of pelletsthrough a first predetermined increment of the distance to thepredetermined depth therein and concurrently rotated relative to thestack, such as in a clockwise direction, at a relatively slow speed.Once the fingers 22 have traveled through the first increment ofdistance, travel is stopped and the fingers 22 pause in their descentwhile they are rotated counterclockwise in the stack at a relativelyfaster speed. Thereafter, their descent into the stack continues for asecond increment of distance as they are again concurrently rotatedslowly. Then, their descent stops and they are rotated faster. Thissequence is again repeated until they reach the desired predetermineddepth. In addition, during at least one interval when the fingers 22 arepausing in their descent, they are rotated several times in oppositedirections to stir the stack of pellets so as to facilitate theirfurther lowering deeper into the stack.

It is preferred that individual pellet samples be removed at thesampling station 10 from different depths in the boat 12 in accordancewith a preprogrammed sampling plan. The number of samples selected andtheir locations and depths can be easily varied at a console (not shown)of the sampling sub-station. The sub-station functions automaticallyinitiating various burrowing and seeking routines until the sample hasbeen retrieved. Sampling points in one exemplary sampling plan areestablished at 3/4, 2 and 31/4 inches from the bottom of the boat 12.

By way of example, one actual sampling sequence is as follows. At thesampling station 10, the manipulator arm 18 moves the gripper mechanism16 to a position above the first programmed sampling point, clear of theboat 12 with the gripping fingers 22 opened and pointing down. Thesampling depth is identified with respect to a "O" reference levelestablished as a preprogrammed distance above the floor of the boat.Then, the gripper mechanism 16 is lowered slowly to a point just abovethe pellet stack (55 mm above "O") and then rotates rapidly 25 degreesin a clockwise direction.

For approaching a first predetermined depth, lowering continues forsuccessive programmed increments (10 mm, 5 mm, 5 mm). As each incrementis approached the gripper mechanism 16 and fingers 22 are rotated slowly25 degrees counterclockwise during the descent, then pauses at eachincrement and rotates rapidly 25 degrees clockwise. In its finalapproach to the first designated depth (30 mm above "O") the grippermechanism 16 lowers an additional increment (5 mm) while rotating slowly25 degrees counterclockwise. If a sample pellet is to be selected atthis first depth, the gripper mechanism 16 executes a grasp action to beexplained shortly.

When directed by the sub-system controller to approach a second deeperpredetermined depth, the robotic manipulator arm 18 causes the grippermechanism 16 to execute the steps required to reach the first depth asjust described and then continues by rotating quickly 25 degreesalternately clockwise and counterclockwise for nine consecutive motionswhile remaining at the same level (30 mm). This stirring action isnecessary to disturb the pellet stack and permit the gripping fingers 22to enter deeper into the stack. Burrowing of the gripping fingersresumes for successive programmed increments (2.5 mm, 2.5 mm) with slow25 degree counterclockwise rotation during descent and rapid 25 degreeclockwise rotation at each increment. Terminating at the seconddesignated depth (25 mm above "O"), the gripper mechanism 16 executes a"grasp" of a sample pellet.

In approaching a third still deeper predetermined depth, the arm 18executes the steps described for the first and second depths andcontinues the normal motion sequence of slow 25 degree clockwiserotation during descent and rapid 25 degree counterclockwise clockwiserotation at each increment level (2.5 mm, 2.5 mm, 2.5 mm, 3.0 mm and 2.5mm), terminating at the third and deepest depth (10 mm above "O") with a"grasp".

If either the preset limit (normally 7 pounds) on the transducer 76 or atime delay is exceeded at any time, the arm 18 ceases the burrowingaction by the gripper mechanism 16 and withdraws 150 mm. The timerautomatically resets and the burrowing cycle is reinitiated. If afterthree attempts the designated depth has not been attained, the armwithdraws 150 mm, an alarm signal is sent to the control room and amessage appears on the CRT.

When the "grasp" command is executed, the air cylinder 34 in the grippermechanism housing 20 is pressurized causing the fingers 22 to close. Ifa pellet is secured between the finger surfaces 32, as will be indicatedby failure of the washer 72 to activate the proximity switch 70, the arm18 then slowly withdraws and proceeds to execute the "shake out"subroutine described below. If the washer 72 has sufficient travel toactivate the proximity switch 70, a signal is provided to the sub-systemcontroller indicating that a pellet has not been secured and the "graspfailed" subroutine is initiated as follows. The controller opens thefingers 22 and withdraws the gripper mechanism 5 mm without rotation.The gripper mechanism 16 is then rotated 50 degrees clockwise, lowered 5mm and rotated 50 degrees counterclockwise simultaneously, lowered 5 mmwithout rotation, raised 5 mm without rotation and then executes "grasp"at the original programmed level. If after seven repetitions a pelletstill has not been secured the gripper mechanism is withdrawn 100 mm, analarm signal is sent to the control room and a message appears on theCRT. This occurrence is generally caused by a partial or grossly unevenboat load.

To remove multiple or extraneous pellets that become entrapped among thefingers 22 (approximately a 5% occurrence) a "shake out" or "invert"sub-routine is employed after the gripper mechanism 16 has moved clearof the boat. The arm 18 is rotated at its "wrist bend" axis such thatthe gripper mechanism 16 is oriented horizontally above the boat; thegripper mechanism is then lowered while in the horizontal position toreduce pellet freefall distance and rotated 90 degrees at the flangeassembly 50 causing unsecured pellets to drop off. The arm 18 thenraises and rotates the gripper mechanism back to a vertical, downwardpointing, orientation clear of the boat. It then swings the grippermechanism to the release position at the pellet orientation station ofthe adjacent density measurement sub-system. The status of the proximitysensor or switch 70 is monitored by the sub-system controller initiallyand again before pellet release to assure that a pellet has been securedand not subsequently dropped. If the pellet is not present for release,a timer causes the cycle to abort and resample.

It is thought that the present invention and many of its attendantadvantages will be understood from the foregoing description and it willbe apparent that various changes may be made in the form, constructionand arrangement thereof without departing from the spirit and scope ofthe invention or sacrificing all of its material advantages, the formhereinbefore described being merely a preferred or exemplary embodimentthereof.

We claim:
 1. A gripper mechanism for locating and grasping a nuclearfuel pellet at a predetermined depth in a stack thereof, comprising:(a)a housing; (b) a plurality of arcuate-shaped gripping fingers mounted tosaid housing and being movable toward and away from respective closedand opened positions for respectively engaging and disengaging thegrasped pellet to correspondingly grasp and release the same, saidgripping fingers having respective lower pellet engaging surfaceportions for grasping said pellet therebetween when said fingers aremoved toward their closed positions, the curvature of said arcuatefingers being sufficient such that other pellets in said stack will notbe grasped therebetween when said gripping fingers engage said graspedpellet; (c) actuating means mounted to said housing and coupled to saidgripping fingers, sand actuating means being operable for moving saidgripping fingers between their closed and opened positions; (d) means,responsive to the movement of the actuating means for detecting whensaid gripping fingers are disposed at their closed position; and (e)means supporting said housing and said gripping fingers mounted theretofor movement toward and away from the stack of pellets to be sampled formoving said gripping fingers into and from the stack, said supportingmeans also resiliently and yieldably mounting said housing so as toallow concurrent lateral rocking of said housing means as said grippingfingers are moved into and from the stack of pellets.
 2. The grippermechanism according to claim 1 wherein said arcuate-shaped grippingfingers are relatively thin and blade-like and said pellet engagingportions have a length less than the length of said pellets.
 3. Thegripper mechanism as recited in claim 1, wherein said detecting means isa proximity switch.
 4. The gripper mechanism as recited in claim 1,further comprising:fingers for sensing the load applied to the stack ofpellets by said gripping means and being capable of generating a signalindicating that an overload condition has been reached.
 5. The grippermechanism as recited in claim 4, wherein said sensing means is aload-sensing transducer.
 6. The gripper mechanism as recited in claim 1,wherein said means resiliently mounting said housing includes:a supportstructure being movable toward and away from the stack of pellets; aplurality of guide members connected to one of said housing and saidsupport structure and slidable relative to the other thereof; and aplurality of coil springs, each coil spring received about one of saidguide members and extending between said housing and said supportstructure, said springs being resiliently yieldable for moving saidhousing and said gripping fingers toward the stack of pellets along agenerally linear path ahead of said support structure as said supportstructure is moved toward the stack, while concurrectly permitting saidhousing to rock about an axis extending transverse to said linear pathfor facilitating burrowing-type movement of said gripping fingers intothe stack of pellets to be sampled.
 7. The gripper mechanism as recitedin claim 6, further comprising:a generally planar transducer mounted toone of said housing and said support structure and extending ingenerally transverse relation to the direction of movement thereof forsensing the load applied to said pellets of said stack by said grippingfingers as the same move within and from said pellet stack, saidtransducer capable of generating a signal indicating that an overloadcondition has been reached.
 8. The gripper mechanism as recited in claim7, further comprising:a generally planar pad mounted to the other ofsaid housing and said support structure and extending in generallytransverse relation to the direction of movement thereof for engagingwith said planar transducer.
 9. The gripper mechanism as recited inclaim 1, wherein said actuating means is a power transmitting cylindermounted to said housing having an extendible and retractible piston rodand a spindle mounted to its terminal end and coupled to said grippingfingers for moving said gripping fingers toward their closed and openedpositions upon retraction and extension of said piston rod into and fromsaid cylinder.
 10. The gripper mechanism as recited in claim 9, whereinsaid detecting means is a proximity switch mounted on said housing fordetecting the position of said piston rod.
 11. A gripper mechanism forgently burrowing into a compact stack of nuclear fuel pellets containedin a boat and grasping a pellet located at a predetermined depth in theboat, comprising:(a) a housing; (b) a plurality of relatively thin,arcuately-shaped gripping fingers movably mounted at their upper ends toand depending from said housing, said fingers being movable betweenclosed and opened positions and having respective lower ends withengaging surfaces facing toward one another, said surfaces being broughtadjacent to one another for engaging and grasping said pellettherebetween when said fingers are moved toward their closed positionand retracted remote from one another for disengaging and releasing saidpellet when said fingers are moved toward their opened position, thecurvature of said arcuate fingers being sufficient such that otherpellets in said stack will not be grasped therebetween when saidgripping fingers engage said grasped pellet; (c) actuating means mountedto said housing and coupled to said upper ends of said gripping fingers,said actuating means being operable for moving said gripping fingersbetween closed and opened positions and a detecting means responsive tothe movement of the actuating means for detecting when said grippingfingers are disposed at their closed positions (d) a support structurebeing movable toward and away from the stack pellets to be sampled; (e)a plurality of guide members connected to one of said housing and saidsupport structure an movable relative to the other thereof; and (f) aplurality of coil springs, each coil spring received about one of saidguide members and extending between said housing and said supportstructure, said springs being resiliently yieldable for moving saidhousing and said gripping fingers toward the stack of pellets along agenerally linear path ahead of said support structure as said supportstructure is moved toward te stack, while concurrently permitting saidhousing to rock slightly about an axis extending transverse to saidlinear path for facilitating a burrowing-type movement of said grippingfingers into the stack of pellets to be sampled.
 12. The grippermechanism as recited in claim 11, further comprising:said detectingmeans being mounted on said housing.
 13. The gripper mechanism asrecited in claim 12, wherein said detecting means is a proximity switch.14. The gripper mechanism as recited in claim 11, furthercomprising:means mounted on said support structure for sensing the loadapplied to the stack of pellets by said fingers and being capable ofgenerating a signal indicating that an overload condition has beenreached.
 15. The gripper mechanism as recited in claim 14, wherein saidsensing means is a load-sensing transducer.
 16. The gripper mechanism asrecited in claim 15, wherein said transducer has a generally planarshape and is mounted to said support structure and extends in generallytransverse relation to the direction of movement thereof.
 17. Thegripper mechanism as recited in claim 16, wherein said housing has agenerally planar pad mounted thereto and extending in generallytransverse relation to the direction of movement thereof for engagingwith said planar transducer.
 18. The gripper mechanism as recited inclaim 11, wherein said actuating means is a power transmitting cylindermounted to said housing having an extendible and retractible piston rodand a spindle mounted to its terminal end and coupled to said upper endsof said gripping fingers for moving said gripping fingers toward andaway from their closed and opened positions upon movement of said pistonrod into and from said cylinder between retracted and extendedpositions.
 19. The gripper mechanism as recited in claim 18, furthercomprising:said detecting means being in the form of a proximity switchmounted on said housing for detecting the presence of said piston rod atits retracted position and thereby the presence of said gripping fingersat their closed position.
 20. The gripper mechanism as recited in claim11, wherein said plurality of gripping fingers are composed of threeblade-like fingers being angularly displaced generally equally from oneanother for grasping the pellet at three equidistantly spaced locationsabout its circumference.